Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 152036

Area Forecast Discussion
National Weather Service Des Moines IA
336 PM CDT Tue Aug 15 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 336 PM CDT Tue Aug 15 2017

Convection over the southeast will be departing prior to 02z.
Newly developing convection over the northwest will continue to
increase this evening with the approaching short wave and zone of
warm air advection over that area. Models continue to have a
difficult time handling the northwest area of convection...though
there is a vague consensus of the area developing north/northeast
through the late evening/overnight while the main trough continues
to increase from the west. Temperatures did not recover fully
over most of the area today due to the persistent cloud
cover...though lows will manage to remain about the same as the
previous forecast due to clouds. The southwest has cleared off for
now...but should see more clouds late into the early morning.
There remains some consensus for increasing PoP/thunderstorms
during the 12z-15z timeframe...though some of the CAMS members and
NAM continue dry until mid to late morning. For now will increase
PoP to chance through 12z and gradually to likely over the
northwest/west during the morning. Confidence is somewhat low
tomorrow morning...but afternoon as the entire trough slides east
the likelihood of showers/thunderstorms will be higher...and PoPs
remain in the likely/definite category. Once again locally heavy
downpours will be possible as PWATs will increase to 1.75 to 2
inches Wednesday afternoon. Have also trimmed highs for Wednesday
across most of the area with the exception of the southeast where
afternoon highs may still make it into the mid 80s. Though
instability will not be excessive tomorrow...there is a potential
for an isolated severe storm with wind being the main concerns.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 336 PM CDT Tue Aug 15 2017

Surface cold front tied to the aforementioned trough will move
across central Iowa Wednesday night with showers and storms
preceding the front. Forecast soundings show precipitable water
(PWAT) values will be well above normal between 1.75 and just over
2 inches across the area with deep warm cloud depths. As the
front moves through, drier air will follow behind with most of the
rainfall ending from west to east by sunrise Thursday as forecast
soundings show dry air in the middle and upper levels of the
atmosphere. As surface high pressure slides eastward over the
Plains, skies will become mostly sunny for Thursday with highs a
few degrees either side of 80.

By Friday, winds from the northwest will turn and blow from the
southwest as a shortwave trough drops out of central Canada through
the Dakotas toward Iowa. This will allow moisture to return back to
the state with PWATs rising from under an inch to around 1.5 inches.
This should allow for some showers and isolated storms to form and
move through later Friday into early Saturday. As the shortwave
exits the area on Saturday, very weak ridging will arrive overhead.
Kept PoPs mainly out of the area until late Sunday when moisture
will begin to move northward as a front approaches from the
northwest. This front looks to be a slow mover as the the
forcing/parent low will be over Canada. Sensible weatherwise, this
means that a mixture of clouds and sunshine with intermittent shower
and storm chances will be possible through early next week. Bottom
line for Monday`s eclipse, it`s still too early to tell one way or
the other on sky cover as timing of the front may change between now
and then.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1253 PM CDT Tue Aug 15 2017

Main concerns are ceilings/vis and convection chances. Decaying
MCS moving slowly east over Iowa now. IFR/MVFR cigs have been slow
to lift over areas of light rain/thunder and expect that afternoon
will remain generally the same. Tonight boundary layer will lower
again with IFR cigs again aft 05-06z north mainly. Will need to
monitor due to convective trends and lingering morning stratus.
Winds generally south southeast through period. Another chance at
thunder nearer KFOD/KMCW toward 12z...though some uncertainty
whether convection west of area overnight will affect any sites
prior to 15z-18z Wednesday as it translates east with time. /rev




LONG TERM...Ansorge
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